1. Field of the Invention
The present invention relates to a field effect transistor device, and in particularly to a heterojunction field effect transistor device which is normally-off.
2. Description of the Related Art
A nitride semiconductor has a high electric breakdown field and a high electron saturation velocity. Thus, the nitride semiconductor is expected to be a semiconductor material for semiconductor devices having a high breakdown voltage and a low on-state resistance. Many of the conventional semiconductor devices using the nitride related materials may have heterojunctions. The heterojunction is configured with two types of nitride semiconductors having different bandgap energies from each other and is able to generate a two-dimensional electron gas layer (2DEG layer) near the junction plane. The semiconductor devices having the heterojunction may achieve a low on-state resistance by passing an electric current through this 2DEG layer. These types of semiconductor devices are called high electron mobility transistors (HEMT).
This type of conventional semiconductor device has a gate portion for switching the electric current. The gate portion has a heterojunction and a gate electrode that faces the heterojunction. A 2DEG layer may be generated near the heterojunction plane with high density. This type of conventional semiconductor device utilizes the 2DEG layer generated near the heterojunction plane as a channel, and as a result, achieves a low on-state resistance. This type of conventional semiconductor device operates in a normally-on mode since there is a source-drain flow of electrons when no voltage is applied to the gate. To turn off the normally-on mode HEMT, a negative voltage is required in order to eliminate the 2DEG layer under the gate electrode. Use of such a negative power supply made the associated circuitry unnecessarily complex and expensive. The conventional normally-on HEMTs are rather inconvenient to use.
Attempts have been made to render the HEMT normally-off. For example, including: partly removing the electron supply layer to expose a part of the electron transit layer and place the gate on the exposed part of the electron transit layer via an insulating film (i.e. gate recess process). However, creating the recess in the electron supply layer leads to shortcomings wherein the threshold voltage changes substantially with manufacturing errors in the depth of the recess in the electron supply layer, and thus the transistor does not have a uniform threshold voltage and has low reliability due to the poor etching tolerance between the electron supply layer and a layer underlying the electron supply layer.
Therefore, a novel field effect transistor device which overcomes the above difficulties and inconveniences is desired